A hot dip galvanizing system which performs hot dip galvanization on a steel sheet, as shown in FIG. 10, is comprised of a coating tank 51 in which molten zinc 71 is filled and a sink roll 52 which is supported by roll support members 53 to hang down inside the coating tank 51 in a rotatable manner. A steel sheet 75 which enters the inside of the coating tank 51 from above is wound around the sink roll 52 whereby it is changed in direction to head upward and is pulled up from the coating tank 51. During this time, the surface of the steel sheet 75 has molten zinc deposited on it whereby a galvanized layer is formed.
If performing such hot dip galvanization, the iron which is eluted from the steel sheet and the molten zinc react whereby bottom dross 72 which is mainly comprised of an iron-zinc alloy is produced and deposits at the bottom of the coating tank 51. In such a hot dip galvanization process, as shown in FIG. 10(B), along with movement of the steel sheet 75 which enters the coating tank 51 from above, a flow in the direction of movement of the steel sheet 75 (below, referred to as a “trailing flow”) is formed in the molten zinc 71 which contacts the steel sheet 75. The trailing flow of molten zinc 71, as shown in FIG. 10(A) runs into a dead end at the position where the steel sheet 75 and the sink roll 52 contact and is discharged to the lateral bottom sides of the sink roll 52. The flow of the molten zinc 71 is reflected at the side walls of the coating tank 51, flows downward, and stirs up the bottom dross 72.
If the bottom dross 72 is stirred up, the stirred up bottom dross 72 deposits on the surface of the steel sheet 75. The bottom dross 72 is hard, so at the time of rolling or working, the surface of the steel sheet 75 is formed with dents as bottom dross defects.
PLT 1 and PLT 2 propose the arts of preventing stir-up of bottom dross 72 and preventing bottom dross defects by providing flow regulating members which cover the bottom or sides of the sink roll 52 and blocking the flow of molten zinc 71 toward the lateral bottom sides of the sink roll 52 by the flow regulating members so as to prevent stir-up of the bottom dross 72.
PLT 3 proposes the art of providing the bottom of a sink roll 52 with a flow regulating member which is provided with a plurality of holes so as to prevent stir-up of the bottom dross 72.